Drive circuit for surface-wave driven motor utilizing ultrasonic vibration

ABSTRACT

A surface-wave driven motor comprises a moving member for being displaced, a resilient member having a surface urged against the moving member, a piezo-electric member provided in contact with the resilient member to impart vibration to the resilient member and create an elastic wave in the surface, means having electrode means provided on the piezo-electric member and applying an AC voltage to the piezo-electric member through the electrode means, means for detecting an electrical output produced in the piezo-electric member from an area on the piezo-electric member which is electrically insulated from the electrode means, and means for determining the frequency of the AC voltage on the basis of the detected electrical output.

.Iadd.This is a continuation of reissue application Ser. No. 083,443filed Aug. 10, 1987, now abandoned. .Iaddend.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a surface-wave driven motor for driving amoving member by an elastic wave of a high frequency.

2. Description of the Prior Art

Any of electric motors heretofore used most generally utilizes anelectromagnetic force and therefore has great limitations in shape andmaterials. Also, they use a winding constituting an electromagnet anduse a permanent magnet of rare earth to obtain a high output, and thishas also led to a limitation in cost.

For such a situation, in recent years, actuators have been developed toreplace electromagnetic type motors, and as one of them, an ultrasonicdriven motor utilizing an ultrasonic wave has been conceived. Thisultrasonic driven motor is regarded as being much more advantageous thanthe conventional motors in shape, efficiency, cost, etc. and as a basicresearch report thereof, IBM Technical Disclosure Bulletin Vol. 16, No.7, etc. have been published from IBM, Inc., U.S.A. As an example of thetrial manufacture of this ultrasonic driven motor, a surface-wave typeultrasonic driven motor (hereinafter referred to the surface-wave drivenmotor) in which a piezo-electric element or the like is used to create asurface wave on the surface of a resilient member and drive a rotorurged against this surface has been devised and already announced.

SUMMARY OF THE INVENTION

It is an object of the present invention to realize a surface-wavedriven motor which utilizes an elastic wave of a high frequency todisplace a moving member.

The surface-wave driven motor of the present invention is provided witha resilient member urged against a moving member, and anelectrostrictive device to which an AC voltage is applied to excite theresilient member, and is characterized in that a voltage created by theexcitation is detected from an area of the electrostrictive device towhich the AC voltage is not applied and the optimum frequency of the ACvoltage to be applied is determined on the basis of the detectedvoltage.

According to a preferred embodiment of the present invention applied toa rotary motor for rotatively displacing a moving member, theelectrostrictive device comprises a piezo-electric member formed into aring-like shape coaxial with a rotor and the piezo-electric member isalternately subjected to polarization treatment so that it has oppositepolarities at predetermined intervals in the circumferential directionthereof. This predetermined length is preferably set to 1/2 of thebending vibration wavelength of the resilient member. An AC voltage isapplied to most of the surface area of the piezo-electric member exceptan area thereof. An output terminal for extracting an alternatingcurrent appearing in response to the bending vibration of thepiezo-electric member is connected to the remaining area of thepiezo-electric member.

The invention will become more fully apparent from the followingdetailed description thereof taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for illustrating the principle of driving of asurface-wave driven motor.

FIG. 2 is a perspective view showing the fundamental structure of thesurface-wave driven motor.

FIGS. 3A and 3B are a plan view and a front view, respectively of asurface-wave driven motor according to an embodiment of the presentinvention.

FIG. 4 is a circuit diagram showing an example of the rotation controldevice for the motor of FIG. 3A.

FIG. 5 is a characteristic curve graph showing the relation of thenumber of revolutions of the motor and the detected voltage to the inputfrequency to the surface-wave driven motor under different extraneousconditions.

FIG. 6 is a graph showing a case where a plurality of peaks appear inthe characteristic curve showing the relation of the number ofrevolutions of the motor and the detected voltage to the inputfrequency.

FIG. 7 is a plan view showing another embodiment of the surface-wavedriven motor according to the present invention.

FIG. 8 is a plan view showing still another embodiment of thesurface-wave driven motor according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principle of driving of the surface-wave driven motor according tothe present invention will hereinafter be described.

When vibration is imparted to the surface of a resilient member by theuse of an electrostrictive device there is created a surface wavepropagated through the surface of the resilient member. In FIG. 1,paying attention to a point A on the surface of an resilient member 1,the point A depicts an elliptical locus. As a result, a moving member 2is driven in the opposite direction (arrow M) to the direction of travel(arrow N) of the surface wave by the friction force between it and theresilient member 1. Accordingly, by forming each of these elements intoan annular shape, there is obtained a rotary drive source. That is, whenvibration is imparted to the resilient member 1 formed in an annularshape as shown in FIG. 2 by the use of an electrostrictive device 3formed by annular ceramics or the like, there is created in the surfaceof the resilient member a surface wave propagated along the annulus andthe moving member 2 is rotated.

A preferred embodiment of the surface-wave driven motor according to thepresent invention which utilizes such a principle will hereinafter bedescribed.

Referring to FIGS. 3A and 3B, a ring-like piezo-electric member 3constituting an electrostrictive device is formed of PZT (zirconic acidtitanic acid lead) or the like. The whole of one surface of thepiezo-electric member 3 is coated with a ring-like electrode 4consisting of silver or the like. The other surface of thepiezo-electric member 3 is coated with a segment electrode group 5consisting of silver or the like. The segment electrode group 5comprises sixteen segment electrodes 501-506. The segment electrode 501has a length of 3/4λ (in the present specification, λ is the bendingvibration wavelength) in the circumferential direction, and the segmentelectrode 502 has a length of 1/4λ in the circumferential direction andis 180° out of phase with the segment electrode 501. The segmentelectrodes 503-509 each have a length of λ/2 in the circumferentialdirection and lie on one side between the segment electrodes 501 and502. On the other hand, the segment electrodes 510-516 each have alength of λ/2 in the circumferential direction and lie on the other sidebetween the electrodes 501 and 502.

The areas of the piezo-electric member 3 in which the segment electrodes503-509 and 510-516 each having the length of λ/2 are opposed to eachother are polarized so that they differ in direction of polarizationfrom the adjacent area. For example, in the piezo-electric member 3, thearea with which the segment electrode 503 is in contact and the areawith which the segment electrode 504 is in contact are opposite indirection of polarization to each other. In FIG. 3A, marks ○.sup.. and ⊕show that the segment electrodes are opposite in direction ofpolarization. The segment electrodes 503-509 each having the length ofλ/2 are electrodes to which a driving alternating current is to beapplied and are connected to one another by electrically conductive past6a, and an input AC voltage is applied to these electrodes from an inputterminal R. The segment electrodes 510-516 also are electrodes to whicha driving alternating current is to be applied and are likewiseconnected to one another by paste 6b, and an input AC voltage is appliedto these electrodes from an input terminal L. The segment electrode 501having the length of 3/4λ is connected to a ring electrode 4 on the backside of the piezo-electric member 3 by electrically conductive paste 6cprovided at the end of the outer periphery of the piezo-electric member3. A ground potential is applied to the segment electrode 501 from agrounded terminal G. That is, this segment electrode 501 merely servesas the leadout electrode of the ring electrode 4. The segment electrode502 is insulated from both of other electrodes 509, 516 adjacent theretoand the ring electrode 4, and does not cooperate with the input ACvoltages from the terminals R and L. this segment electrode 502 isprovided with an output terminal S, and the area of the piezo-electricmember 3 which is opposed to the electrode 502 is polarization-treated.The ring-like piezo-electric member 3 has the ring-like resilient member1 of brass or the like secured to the side thereof which is adjacent tothe ring electrode 4 by an adhesive agent 7. The rotor 2 bears againstthe resilient member 1 with a certain contact pressure.

When AC voltages of a predetermined frequency which are 90° out of phasewith each other are applied between the input terminals R and G andbetween the input terminals L and G, bending vibration of wavelength λis created in the piezo-electric member 3 and the resilient member 1 andthe rotor 2 is rotated. To enhance the efficiency of this motor, it isnecessary to keep the frequency of said AC voltages at an optimum value.However, this optimum value is variable by extraneous conditions such asthe contact pressure between the rotor 2 and the resilient member 1 andthe temperature of the motor. Accordingly, if the frequency of said ACvoltages is always kept at a predetermined value, there cannot beobtained a surface-wave driven motor of high efficiency.

Therefore, in the present embodiment, provision is made of specificmeans as described hereinafter which utilizes the output from theterminal S to maintain the frequency of the input power source always atan optimum value conforming to the extraneous conditions.

The segment electrode 502 to which the terminal S is connected is anelectrode to which the input AC voltage is not applied and, when themotor is driven and bending vibration is created in the piezo-electricmember 3, an alternating current corresponding to this bending vibrationappears in the segment electrode 502. This phenomenon is due to theproperty of the piezo-electric member which creates vibration if analternating current is applied thereto and which creates an AC output ifvibration is imparted thereto.

The AC output from the terminal S is applied as an input to a rotationcontrol device shown in FIG. 4. The rotation control device comprises acontrollable oscillator 101, a 90° phase shifter 102, amplifiers 103,104, inductances 105, 106, a band-pass filter 107 and an oscillationcontroller 108. A sine wave of a certain frequency is put out from theoscillator 101 and applied as an input to the amplifier 103 and thephase shifter 102. The output of the phase shifter 102 is applied as aninput to the amplifier 104. The amplifiers 103 and 104 apply AC voltages90° out of phase with each other to the surface-wave driven motor 100from the terminals R and L through the inductances 105 and 106. Thestate of revolution of the surface-wave driven motor is put out in theform of a magnitude of AC voltage from the terminal S connected to thesegment electrode 502 and is applied as an input to the band-pass filter107. Any unnecessary signal is removed by the band-pass filter 107 andonly a signal necessary for controlling the revolution is applied as aninput to the oscillation controller 108 and fed back to the controllableoscillator 101.

FIG. 5 is a characteristic curve graph showing the relation when thenumber of revolutions of the surface-wave driven motor is N for theinput frequency f and the voltage (for example, the actually effectivevalue) detected from the terminal S of the segment electrode 502 fordetecting the vibration of the surface-wave driven motor at that time isV. In FIG. 5, the characteristic of the number of revolutions of themotor and the characteristic of the detected voltage for the inputfrequency under certain extraneous conditions are as indicated by N₁ andV₁, respectively, and both of them become maximum at an optimum inputfrequency f₁. Also, when the extraneous conditions have changed, thecharacteristic of the number of revolutions of the motor and thecharacteristic of the detected voltage for the input frequency are asindicated by N₂ and V₂, respectively, and at this time, N₂ and V₂ aremaximum for the optimum input frequency f₂. In this manner, it has beenempirically found that under certain extraneous conditions, thefrequency which gives a peak to the number of revolutions N of the motorand the frequency which gives a peak to the detected voltage V aresubstantially coincident with each other. If, in spite of a change inthe extraneous conditions, the input frequency is still maintained, forexample, at f₁ , both the number of revolutions N of the motor and thedetected voltage V will be greatly reduced. So, the rotation controldevice of FIG. 4 is designed such that the oscillation controller 108controls the oscillator 101 in accordance with the voltage detected fromthe terminal S of the segment electrode 502 of the surface-wave drivenmotor and always controls the oscillation frequency i.e., the inputfrequency to the surface-wave driven motor, at the optimum value (e.g.f₂) of the then extraneous conditions. Thus, even if the resonancefrequency between the ring-like piezo-electric member and the resilientmember in the surface-wave driven motor is varied under the influence ofdisturbance, an optimum frequency is always put out from the oscillator101 and accordingly, the revolution of the surface-wave driven motor ismaintained optimum.

On the other hand, in some cases, the voltage V detected from theterminal S connected to the segment electrode 502 of the surface-wavedriven motor, as previously described, assumes a maximum value at theinput frequency to the motor which provides the maximum value of thenumber of revolutions N of the motor and, as shown in FIG. 6, presents asmall peak at a frequency f₃ considerably distant therefrom. To hamperthe adverse effect of this frequency f₃, the band-pass filter 107 passestherethrough a frequency band which provides the maximum value of thenumber of revolutions N of the motor, for example, the vicinity of thefrequencies f₁ -f₂ of FIG. 5, and cuts the other frequency bandsincluding the above-mentioned frequency f₃.

If, as in the above-described embodiment, the resilient member is formedinto a ring-like shape, the resonance frequency thereof differs in theinside and the outside of the ring. Also, the surface wave which drivesthe rotor concentrates on the outside of the ring. Accordingly, amonitor voltage introduced into the rotation control device shoulddesirably be one which has detected the vibration of the outer peripheryof the ring of the piezo-electric member. Thus, by coating only theouter peripheral portion of the ring-like piezo-electric member with avibration detecting segment electrode, vibration in the outer peripheralportion of the ring-like piezo-electric member can be detected.

In the embodiment shown in FIG. 7, an electrically insulated segmentelectrode 521 is formed in the outer periphery of an area on thepiezo-electric member between segment electrodes 509 and 516 which has alength of 1/4λ in the circumferential direction, and a monitor voltagedetecting output terminal S is connected to this segment electrode 521.

Also, as a monitor voltage detecting electrode, for example, anelectrode 501 is divided radially to form two mutually insulatedelectrodes 522 and 523 as shown in FIG. 8, and the inner electrode 522is connected to the ring-like electrode 4 by electrically conductivepaste 6d and also connected to the grounded terminal G so that it may beused as the lead-out electrode of the ring-like electrode, and theterminal S is connected to the outer electrode 523 so that thiselectrode may be used as a vibration detecting electrode. Likewise, anyof segment electrodes 503-509 and 510-516, for example, electrode 507,can be divided to form a driving AC applying electrode and a vibrationdetecting electrode.

As a modification of the present invention, it is also possible toinsulate, for example, a small portion of the ring-like electrode 2 fromthe remainder thereof and use this small portion as a vibrationdetecting electrode and on the other hand, insulate the portion of thesegment electrode group 5 which is opposed to the vibration detectingelectrode with the piezo-electric member 3 interposed therebetween fromthe other portion and use this portion as the opposed electrode of thevibration detecting electrode and take out a vibration detection signalfrom the vibration detecting electrode and the opposed electrode. Ofcourse, this opposed electrode may be one which corresponds to thesegment electrode 501 or 502 to which the AC voltage is not applied.

In the above-described example, the vibration detecting electrode andthe opposed electrode which are opposed to each other with thepiezo-electric member interposed therebetween are used as a pair ofelectrodes for detecting vibration, but this pair of electrodes may bothbe provided on the side of the ring-like electrode 4 or on the side ofthe segment electrode group 5. Specifically, for example, a pair ofadjacent electrodes insulated from the other electrodes may be providedon any one of the surfaces of the piezo-electric member so that both ofthem are positioned in the same polarized area of the piezo-electricmember.

We claim: .[.1. A surface-wave driven motor comprising: direction inwhich said moving member is displaced..].
 4. A surface-wave driven motor.[.according to claim 1,.]. .Iadd.comprising:a moving member for beingdisplaced; a resilient member having a surface urged against said movingmember; a piezo-electric member provided in contact with said resilientmember to impart vibration to said resilient member and create anelastic wave in said surface; means having electrode means provided onsaid piezo-electric member and applying an AC voltage to saidpiezo-electric member through said electrode means; means for detectingan electrical output produced in said piezo-electric member from an areaon said piezo-electric member which is electrically insulated from saidelectrode means; and means for determining the frequency of said ACvoltage on the basis of the detected electrical output, .Iaddend.whereinsaid detecting means includes means for selecting an AC output which iswithin a predetermined frequency range created in said piezo-electricmember. .[.5. A surface-wave driven motor according to claim 1, whereinsaid detecting means detects an AC voltage produced in saidpiezo-electric member, and said determining means includes means forcontrolling the frequency of the AC voltage of said applying means at afrequency whereat the detected AC voltage becomes maximum..]. .[.6. Amotor provided with a rotor rotated by a surface wave, comprising: anannular resilient member disposed with its surface being in contact withsaid rotor; an annular piezo-electric member provided in contact withsaid resilient member to impart bending vibration to said resilientmember; means having electrode means provided on said piezo-electricmember and applying an AC voltage to said piezo-electric member throughsaid electrode means; means for detecting an electrical output producedin said piezo-electric member from an area on said piezo-electric memberwhich electrically insulated from said electrode means; and means fordetecting the frequency of said AC voltage on the basis of the detectedelectrical output..]. .[.7. A motor according to claim 6, wherein saidpiezo-electric member is subjected to polarization treatment so that thepolarity thereof is inverted at predetermined intervals in thecircumferential direction..]. .[.8. A motor according to claim 6,wherein said detecting means includes another electrode meanselectrically insulated from said electrode means of said applying meansand provided on said piezo-electric member..]. .[.9. A motor accordingto claim 8, wherein said another electrode means is disposed at aposition adjacent to the outer peripheral surface of the annulus of saidpiezo-electric member..]. .[.10. A motor according to claim 6, whereinsaid rotor, said resilient member and said piezo-electric member arecoaxial with one another with respect to the axis of rotation of saidrotor and axially contact one another..]. .Iadd.11. A surface-wavedriven motor comprising:a moving member for being displaced; a resilientmember having a surface urged against said moving member; apiezo-electric member provided in contact with said resilient member toimpart vibration to said resilient member and create an elastic wave insaid surface; means having electrode means provided on saidpiezo-electric member and applying an AC voltage to said piezo-electricmember through said electrode means; means for detecting an electricaloutput produced in said piezo-electric member from an area on saidpiezo-electric member which is electrically insulated from saidelectrode means, said detecting means including an electrode on saidarea; and means for determining the frequency of said AC voltage on thebasis of the detected electrical output, said determining meansincluding means for controlling the frequency of said AC voltage inresponse to said detected electrical output each time said electricaloutput varies throughout said applying of said AC voltage. .Iaddend..Iadd.12. A surface-wave driven motor according to claim 11, whereinsaid piezo-electric member is subjected to polarization treatment sothat the polarity thereof is inverted at predetermined intervals in adirection in which said moving member is displaced. .Iaddend. .Iadd.13.A surface-wave driven motor according to claim 11, wherein saidpiezo-electric member is subjected to polarization treatment so that thepolarity thereof is inverted at each length of about 1/2 of a bendingvibration wavelength created in said resilient member in a direction inwhich said moving member is displaced. .Iaddend. .Iadd.14. Asurface-wave driven motor according to claim 11, wherein said detectingmeans detects an AC voltage produced in said piezo-electric member, andsaid controlling means adjusts the frequency of the AC voltage of saidapplying means to a frequency whereat the detected AC voltage becomesmaximum. .Iaddend. .Iadd.15. A surface-wave driven motor comprising:amoving member for being displaced; a resilient member having a surfaceurged against said moving member; a piezo-electric member provided incontact with said resilient member to impart vibration to said resilientmember and create an elastic wave in said surface; means havingelectrode means provided on said piezo-electric member and applying anAC voltage to said piezo-electric member through said electrode means todrive said motor; the efficiency of said motor being dependent upon thefrequency of said AC voltage, and optimum frequency of said AC voltagefor enhancement of said motor efficiency being variable with change inan extraneous condition such as the pressure between said moving memberand said resilient member and the temperature of said motor; means fordetecting, during driving of said motor, an electrical output producedin said piezo-electric member from an area on said piezo-electric memberwhich is electrically insulated from said electrode means; and means fordetermining the frequency of said AC voltage on the basis of thedetected electrical output, said determining means including means forcontrolling the frequency of said AC voltage in response to saidelectrical output each time said electrical output varies throughout thedriving of said motor, and for adjusting the frequency of said ACvoltage to provide an optimum frequency. .Iaddend. .Iadd.16. Asurface-wave driven motor comprising; a moving member for beingdisplaced; a resilient member having a surface urged against said movingmember; a piezo-electric member provided in contact with said resilientmember to impart vibration to said resilient member and create anelastic wave in said surface; means having electrode means provided onsaid piezo-electric member and applying an AC voltage to saidpiezo-electric member through said electrode means; means for detectingan electrical output produced in said piezo-electric member from an areaon said piezo-electric member which is electrically insulated from saidelectrode means; means for determining the frequency of said AC voltageon the basis of the detected electrical output; and filtering meansprovided between said detecting means and said determining means to passtherethrough a predetermined frequency band and to impede a frequencyband other than said predetermined frequency band. .Iaddend. .Iadd.17. Asurface-wave driven motor according to claim 16, wherein said electricaloutput includes two peak voltage values at two frequencies,respectively, of said AC voltage, only one of said peak values being amaximum peak value, and wherein said predetermined frequency bandincludes said maximum peak value and excludes the other of said two peakvalues. .Iaddend. .Iadd.18. A surface-wave driven motor according toclaim 16, wherein said determining means includes means for adjustingthe frequency of the AC voltage of said applying means to a frequencywhereat the detected electrical output passed through said filteringmeans becomes maximum. .Iaddend. .Iadd.19. A surface-wave driven motoraccording to claim 16, wherein said piezo-electric member is subjectedto polarization treatment so that the polarity thereof is inverted atpredetermined intervals in a direction in which said moving member isdisplaced. .Iaddend. .Iadd.20. A surface-wave driven motor according toclaim 16, wherein said piezo-electric member is subjected topolarization treatment so that the polarity thereof is inverted at eachlength of about 1/2 of a bending vibration wavelength created in saidresilient member in a direction in which said moving member isdisplaced. .Iaddend. .Iadd.21. A surface-wave driven motor comprising;amoving member for being displaced; a resilient member having a surfaceurged against said moving member; a piezo-electric member provided incontact with said resilient member to impart vibration to said resilientmember and create an elastic wave in said surface; means havingelectrode means provided on said piezo-electric member and applying anAC voltage to said piezo-electric member through said electrode means;means for detecting an electrical output produced in said piezo-electricmember from an area on said piezo-electric member which is electricallyinsulated from said electrode means, said electrical output assuming amaximum peak value for a predetermined frequency of said AC voltage andassuming a small peak value for a frequency considerably distant fromsaid predetermined frequency; means for determining the frequency ofsaid AC voltage on the basis of the detected electrical output; andfiltering means provided between said detecting means and saiddetermining means to pass therethrough a predetermined frequency bandincluding said predetermined frequency and to impede a frequency bandincluding said distant frequency. .Iaddend. .Iadd.22. A surface-wavedriven motor comprising; a moving member for being displaced; aresilient member having a surface urged against said moving member; apiezo-electric member provided in contact with said resilient member toimpart vibration to said resilient member and create an elastic wave insaid surface; means having electrode means provided on saidpiezo-electric member and applying an AC voltage to said piezo-electricmember through said electrode means; means for detecting an electricaloutput produced in said piezo-electric member from an area on saidpiezo-electric member which is electrically insulated from saidelectrode means, said detecting means including an electrode on saidarea; means for determining the frequency of said AC voltage on thebasis of the detected electrical output, said determining meansincluding means for controlling the frequency of said AC voltage inresponse to said electrical output each time said electrical outputvaries throughout said applying of said AC voltage; and filtering meansprovided between said detecting means and said determining means to passtherethrough a predetermined frequency band and to impede a frequencyband other than said predetermined frequency band. .Iaddend. .Iadd.23. Asurface-wave driven motor comprising;a moving member for beingdisplaced; a resilient member having a surface urged against said movingmember; a piezo-electric member provided in contact with said resilientmember to impart vibration to said resilient member and create anelastic wave in said surface; means having electrode means provided onsaid piezo-electric member and applying an AC voltage to saidpiezo-electric member through said electrode means to drive said motor;the efficiency of said motor being dependent upon the frequency of saidAC voltage, an optimum frequency of said AC voltage for enhancement ofsaid motor efficiency being variable with change in an extraneouscondition such as the pressure between said moving member and saidresilient member and the temperature of said motor; means for detecting,during driving of said motor, an electrical output produced in saidpiezo-electric member from an area on said piezo-electric member whichis electrically insulated from said electrode means, said electricaloutput assuming a maximum peak value for an optimum frequency of said ACvoltage and assuming a small peak value for a frequency considerablydistant from an optimum frequency; means for determining the frequencyof said AC voltage on the basis of the detected electrical output, saiddetermining means including means for controlling the frequency of saidAC voltage in response to said electrical output each time said optimumsaid electrical output varies throughout said driving of said motor, andfor adjusting the frequency of said AC voltage to provide an optimumfrequency; and filtering means provided between said detecting means andsaid determining means to pass therethrough a predetermined frequencyband including an optimum frequency and to impede a frequency bandincluding said distant frequency. .Iaddend. .Iadd.24. A motor providedwith a rotor rotated by a surface wave, comprising; an annular resilientmember disposed with its surface being in contact with said rotor; anannular piezo-electric member in contact with said resilient member toimpart bending vibration to said resilient member, said annularpiezo-electric member comprising a ring having at one side thereof aninner peripheral area and an outer peripheral area that is positionedfarther from the center of the ring than said inner peripheral area;means having electrode means provided on said piezo-electric member andapplying an AC voltage to said piezo-electric member through saidelectrode means; means for detecting an electrical output produced insaid piezo-electric member from an area on said piezo-electric memberwhich is electrically insulated from said electrode means, saiddetecting means having an electrode provided at said outer peripheralarea but not at said inner peripheral area; and means for determiningthe frequency of said AC voltage on the basis of the detected electricaloutput. .Iaddend. .Iadd.25. A surface-wave driven motor according toclaim 24, wherein said electrode means includes a portion at said innerperipheral area at a sector of said annular piezo-electric member thatis common to said electrode of said detecting means. .Iaddend. .Iadd.26.A motor provided with a rotor rotated by a surface wave, comprising:anannular resilient member disposed with its surface being in contact withsaid rotor; an annular piezo-electric member provided in contact withsaid resilient member to impart bending vibration to said resilientmember, said piezo-electric member comprising a ring having a pluralityof sections which are arranged in a circumferential direction of saidring and which include first and second sections spaced from each otherand a third section positioned between said first and second sections,said third section having a length of about 1/4 of a wavelength of saidbending vibration in the circumferential direction; means havingelectrode means provided on said first and second sections of saidpiezo-electric member and applying an AC voltage to said piezo-electricmember through said electrode means; means for detecting an electricaloutput produced in said piezo-electric member from an area on saidpiezo-electric member which is electrically insulated from saidelectrode means, said area including said third section of saidpiezo-electric member and having an electrode on said third section fordetecting an electrical output produced in said third section; and meansfor determining the frequency of said AC voltage on the basis of thedetected electrical output. .Iaddend. .Iadd.27. A motor according toclaim 26, wherein said third section lies between first ends of saidfirst and second sections, and wherein said plurality of sections ofsaid piezo-electric member further includes a fourth section lyingbetween second ends of said first and second sections and having alength of about 3/4 of a wavelength of said bending vibration in thecircumferential direction. .Iaddend. .Iadd.28. A motor according toclaim 26, wherein said determining means includes means for controllingthe frequency of said AC voltage in response to said electrical outputdetected by said detecting means each time said electrical output variesthroughout said applying of said AC voltage. .Iaddend. .Iadd.29. Asurface-wave driven motor comprising;a moving member for beingdisplaced; a resilient member having a surface urged against said movingmember; a piezo-electric member provided in contact with said resilientmember to impart vibration to said resilient member and create anelastic vibration wave in said surface that is propagated along saidsurface; means having electrode means provided on said piezo-electricmember and applying an AC voltage to said piezo-electric member throughsaid electrode means; means for detecting an electrical output producedin said piezo-electric member from an area on said piezo-electric memberwhich is electrically insulated from said electrode means; and means fordetermining the frequency of said AC voltage on the basis of thedetected electrical output; wherein said means having electrode meansincludes first, second, and third electrodes electrically insulated fromeach other and provided on one side of said piezo-electric member, and afourth electrode provided on the opposite side of said piezo-electricmember between said piezo-electric member and said resilient member,electrically conductive paste means extending from said third electrodeto said fourth electrode over a periphery of said piezo-electric memberin order to electrically connect said fourth electrode to said thirdelectrode, and means for applying first and second AC voltages which areout of phase with each other between said first and third electrodes,and between said second and third electrodes, respectively. .Iaddend..Iadd.30. A surface-wave driven motor according to claim 29, whereinsaid means having electrode means further comprises first, second, andthird terminals on said first, second, and third electrodes,respectively, and wherein said first and second AC voltages are appliedbetween said first terminal and said third terminal, and between saidsecond terminal and said third terminal, respectively. .Iaddend..Iadd.31. A surface-wave driven motor according to claim 29, whereinsaid resilient member and said piezo-electric member are annular andsaid third electrode has a length of 3/4 wavelength of said vibrationwave in a circumferential direction of said piezo-electric member..Iaddend. .Iadd.32. A surface-wave driven motor according to claim 2,wherein said piezo-electric member is ring-shaped and said electricallyconductive paste means is coated on an outer periphery of saidpiezo-electric member. .Iaddend. .Iadd.33. A surface-wave driven motorcomprising:a moving member for being displaced; a resilient memberhaving a surface urged against said moving member; a piezo-electricmember provided in contact with said resilient member to impartvibration to said resilient member and create an elastic vibration wavein said surface that is propagated along said surface; means havingelectrode means provided on said piezo-electric member and applying anAC voltage to said piezo-electric member through said electrode means;means for detecting an electrical output produced in said piezo-electricmember from an area on said piezo-electric member which is electricallyinsulated from said electrode means; and means for determining thefrequency of said AC voltage on the basis of the detected electricaloutput; wherein said means having electrode means includes first andsecond groups of electrodes provided on one side of said piezo-electricmember, and an electrode member provided on the opposite side of saidpiezo-electric member between said piezo-electric member and saidresilient member, each of said electrodes has a length of 1/2 wavelengthof said vibration wave and said electrode member as a length longer thansaid each electrode, and means for applying first and second AC voltageswhich are out of phase with each other between said first group ofelectrodes and said electrode member and between said second group ofelectrodes and said electrode member, respectively. .Iaddend. .Iadd.34.A surface-wave driven motor according to claim 33, wherein saidresilient member, said piezo-electric member, and said electrode memberare annular. .Iaddend. .Iadd.35. A surface-wave driven motorcomprising:a moving member for being displaced; a resilient memberhaving a surface urged against said moving member; a piezo-electricmember provided in contact with said resilient member to impartvibration to said resilient member and create an elastic vibration wavein said surface that is propagated along said surface; means havingelectrode means provided on said piezo-electric member and applying anAC voltage to said piezo-electric member through said electrode means;means for detecting an electrical output produced in said piezo-electricmember from an area on said piezo-electric member which is electricallyinsulated from said electrode means; and means for determining thefrequency of said AC voltage on the basis of the detected electricaloutput; wherein said means having electrode means includes first andsecond electrode members each of which has a plurality of electrodesprovided on one side of said piezo-electric member, a third electrodemember provided on the opposite side of said piezo-electric memberbetween said piezo-electric member and said resilient member,electrically conductive paste means for electrically connecting saidplurality of electrodes of said first electrode member with one anotherand electrically connecting said plurality of electrodes of said secondelectrode member with one another, and means for applying first andsecond AC voltages which are out of phase with each other between saidfirst electrode member and said third electrode member and between saidsecond electrode member and said third electrode member, respectively..Iaddend. .Iadd.36. A surface-wave driven motor according to claim 35,wherein said detecting means includes a fourth electrode member providedon said one side of said piezo-electric member between said first andsecond electrode members. .Iaddend.